This invention relates to active matrix electroluminescent display devices, comprising an array of electroluminescent display pixels arranged in rows and columns. The invention is particularly concerned with display devices in which rows of pixels share a common line, with currents through the display elements of the row passing along the common line.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer.
The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display elements. Examples of an active matrix electroluminescent display are described in U.S. Pat. No. 5,67,0792, the contents of which are incorporated herein by way of reference material.
A problem with display devices of this type arises from the fact that they have current driven display elements. Display devices of the type with which this invention is concerned include a common line on which the currents from all pixels in a row pass. Compounding currents from the pixels in a row give rise to different voltages along the common line. These voltages depend upon the currents through all pixels in the row, since these currents all pass to the common line. These different voltages give rise to undesired changes to the outputs from the display pixels, which vary as a function of the full set of signals applied to the row. Consequently, there is cross-talk between the pixels within the row.
According to the invention, there is provided an active matrix electroluminescent display device comprising:
an array of display pixels arranged in rows and columns, each pixel comprising an electroluminescent display element and a switching means for controlling the current through the display element based on a signal voltage applied to the pixel, each row of pixels sharing a common line, currents through the display elements of a row of pixels passing along the common line; and
driver circuitry for generating signal voltages corresponding to desired outputs from the display elements, and for applying signal voltages to rows of pixels in sequence, wherein the device further comprises:
means for generating error values for each pixel in a row of pixels to be addressed, derived from a circuit representing an additional row of pixels and associated with the driver circuitry, and to which the signal voltages for the row of pixels to be addressed are applied;
means for updating the signal voltages for each pixel in the row of pixels to be addressed, where required, using the error values; and
means for supplying the updated signal voltages to the pixels.
The display device of the invention includes a circuit representing an additional row of pixels, so that the signal voltages can be applied to the additional row, and the actual outputs from the display elements can be evaluated, to enable a discrepancy between the desired output and the actual output to be corrected. This discrepancy arises from the different voltages on the common line at each pixel, which depend upon the signal voltages for the row of pixels. The device of the invention can thereby compensate each pixel individually for signal distortions resulting from cross-talk between pixels within a row.
The voltages on the common line of the additional row of pixels, at the position of each pixel, may be supplied as the error signals to a combining element which updates the signal voltages. This arrangement specifically corrects for the resistance of the common signal line, which gives rise to the cross-talk described above.
The display element and the switching means are preferably arranged in series between a voltage supply line for the display elements and the common line, which acts as a current drain.
Each pixel may further comprise a charge storage element for holding a control voltage derived from the updated signal voltage on the switching means.
The invention also provides a method of driving an active matrix electroluminescent display device, comprising an array of electroluminescent display pixels arranged in rows and columns, each row of pixels sharing a common line, currents through the display pixels of a row passing along the common line, different voltages thereby being present at different points along the common line, the method comprising addressing rows of pixels in sequence, and for each row of pixels generating voltage signals for the pixels in the row, the generated voltage signals corresponding to desired pixel outputs, wherein the method further comprises:
generating error values based on the effect of the voltage signals on a circuit representing an additional row of pixels;
updating the signal voltages for each pixel in the row, where required, using the error values; and
supplying the updated signal voltages to the pixels.